In recent years, in a welding step of welding automotive components or the like, a resistance spot welding method is widely used in which two steel plates are overlapped and then sandwiched by a pair of electrodes, and the overlapped body is then energized while being pressed so as to form a molten pool (usually called “nugget”) at an interface between the steel plates and thereby to join the steel plates.
For example, a door opening portion of an automobile includes, as structural members, a pillar and a roof rail. A pillar 20 (see, for example, FIG. 4) includes an overlapped body 21 in which, for example, two steel plates are overlapped. As shown in FIG. 4, the overlapped body 21 is joined by forming weld portions 23 at a predetermined interval by spot welding in a flange 22 of the overlapped body 21.
As the overlapped body described above, various combinations (hereinafter referred to as “plate combinations”) can be conceived by selecting the material of the two steel plates. At the time of performing resistance spot welding, the welding pressure (pressing force) and the amount of energization are set so as to be appropriate for each plate combination. With some plate combinations, it may be difficult to attain a sufficient range (hereinafter referred to as “proper current range”) of welding current (hereinafter referred to as “proper current”) that can provide a sound nugget without causing erosion called expulsion and surface flash (also called spatter, splash or the like). As used herein, “sound nugget” refers to a nugget having a sufficiently large molten pool and exhibiting a sufficient joint strength in a tensile test or the like using a joined body (joint). “Sufficiently large molten pool” refers to a molten pool having a diameter larger than, for example, 4√t (mm) (where t is the thickness (mm) of the thinner one of the two steel plates constituting a plate combination). The proper current range is determined based on the conditions determined by equipment and production constraints or the like (combined conditions of welding pressure and energization time), and can be determined as the difference between the upper limit value of the proper current (hereinafter referred to as “upper limit current”) and the lower limit value of the proper current (hereinafter referred to as “lower limit current”). It is generally believed that the proper current range is desirably wide because stable welding quality can be attained even if disturbance (current fluctuations and the wear of electrode tip or the like) occurs during welding.
Generation of the expulsion and surface flash during welding will degrade the work environment and will cause a reduction in product quality with adhesion of spatters onto the product surface. Furthermore, if an excessively large amount of the expulsion and surface flash is generated, the volume of a fusion joined portion will decrease, which significantly reduces the joint strength in the joined portion. For the reasons given above, it is considered desirable to suppress the generation of the expulsion and surface flash to the extent possible.
Under the circumstances, Patent Documents 1, 2, 3 and 4 disclose techniques for increasing the diameter of nuggets while suppressing the generation of the expulsion and surface flash by improving the conformance (contact state) of the contact plane between steel plates so as to attain a sufficient contact area during energization. These techniques can be construed as techniques that can raise the upper limit current.
Patent Document 5 discloses a technique for increasing the area of a corona bond (a ring shaped portion formed around a nugget by solid state welding, see JIS Z 3001-6 2013) by pressing the periphery of a contact portion between a spot welding electrode and a material to be welded by an insulation indenter. Patent Document 5 teaches that as a result of increasing the area of a corona bond, the same effects as those obtained by increasing the nugget diameter can be obtained. This technique can be construed as a technique that can prevent the generation of the expulsion and surface flash by suppressing the welding current to a low level.
In a welding step of welding automotive components or the like with the use of resistance spot welding, generally, a plurality of welding spots are provided consecutively at locations required from the design point of view. Accordingly, when resistance welding is performed at a given location, if there already is a welding spot near the location (hereinafter referred to as “existing welding spot”), a branch current that flows through the existing welding spot as an energization path is generated. Another case is also conceived in which an energization path is formed at a location other than the existing welding spot and a branch current is generated depending on the geometric shape of members and the arrangement of space with another member. As described above, if the welding current is branched at the time of welding, formation of the molten pool is delayed and thus a sound nugget cannot be obtained. The branch current is also called reactive current, and various investigations have been made on the method for limiting the influence thereof.
For example, Patent Document 6 discloses an invention in which a reactive current is calculated and a current that is increased by an amount corresponding to the calculated reactive current is set as a welding current. Patent Document 7 discloses a method for obtaining a sound nugget by forming a slit so as to reduce the influence of reactive current.